Conventionally, for example, with a bottom trawl fishing method, the obtaining of information relating to a fishing net "a" being dragged by a ship "d" such as a range from the fishing net "a" to the sea-bottom "b", a range from the net "a" to the water surface "c" and fish schools moving around the net, is shown in FIG. 4. In order to obtain the information, conventionally, a fish school detecting apparatus has been used as shown in FIG. 5.
With a conventional fish school detecting apparatus, a transmitter "g" is fixed at an upper portion of the mouth of the fishing net "a"; a receiver "h" is installed on the ship "d"; and further, the transmitter "g" is provided with two transmitting and receiving transducers "i1" and "i2" to produce echo signals obtained by radiating ultrasonic wave signals in up and down directions and to receive resultant echo signals therefrom respectively. The fishing net "a" is being dragged by the ship "d", and at the same time ultrasonic wave signals are emitted and echo signals are received alternately at time intervals by the transmitting and receiving transducers "i1" and "i2" at a fishing net side. Echo signals coming from the up and down directions with respect to the fishing net "a" and received thereby are supplied through an amplifier "j" to an AM modulator "k" (or an FM modulator) which modulates in amplitude the echo signals (or in frequency). The modulated echo signals are amplified by a power amplifier "m" and then are converted by means of a transmission transducer "n" to ultrasonic wave signals which propagate to a ship side. The signals transmitted from the net are received by a receiving transducer "p" of a receiver "h" installed on the ship "d". The echo signals received thereby are amplified by an amplifier "q" and then recorded by a recorder or displayed by a color monitor. The recorder and the color monitor are not shown.
The frequency bandwidth in relation to ultrasonic wave signals transmitted from a transmitter to a receiver is generally of a narrow frequency bandwidth determined by characteristics of an ultrasonic transducer. In addition, another limitation is imposed by a requirement that there has to be avoided mutual interferences with signals used by other ships and hence the frequency bandwidth can not be too broad.
With a bottom trawl fishing method, information in association with areas above and below of the net and in the neighborhood of the fishing net is most important. The farther a position goes away from the fishing net, the less the importance of information associating the point becomes. In other words, there is desired to obtain detailed information in association with narrow areas in the neighborhood of the fishing net (for example, bandlike areas above and below the fishing net, each having a vertical length of about 5 meter). There is not desired to obtain information in association with positions considerably far away from the ship, which is, however, desired to be obtained by scanning sonars and the like.
With prior art underwater detection apparatii, information produced in the neighborhood of the fishing net and obtained and other information produced in areas farther away therefrom are transmitted at the same information density. From a viewpoint of information quality, the necessary information is inadequate and poor in association with the neighborhood of the transmitting and receiving transducer, and information produced in areas farther away from the transducers is excessive.
Thus, prior art apparatii have presented such drawbacks that record or display of necessary information becomes obscure due to inadequacy of the information produced in the neighborhood as well as narrow signal frequency bandwidth explained in the foregoing, when the information in association with the neighborhood of the fishing net is displayed or recorded.
The present invention is created in order to solve the drawbacks. An object of the present invention is to clearly record or display the information produced in the neighborhood of the fishing net by using the same narrow transmission bandwidth as with prior art apparatii so that signal interference with signals transmitted by other ships is not realized and by transmitting the information in association with the neighborhood of the fishing net at an increased density of information.